Backers of Republican Rep. Mike Simpson Step Up Spending to Foil Attempt to Oust the Incumbent

IDAHO FALLS, Idaho—Tea-party groups trying to push the Republican Party to the right picked their biggest House fight of the year here, in one of the most conservative districts in the country. But the battle isn't going the way they imagined.

Instead of showing its power, the tea-party effort to oust Rep. Mike Simpson in a GOP primary is becoming a symbol of the likely limits of the movement and of the strength of pro-business...